Research work uncovers soaring scab incidence
By Marianne Curtis
SCAB incidence has increased dramatically since the disease ceased to be notifiable in 1992.
Lack of a cohesive control policy, inadequate biosecurity and misunderstandings over treatments are largely to blame, says the author of a recent study.
Concern for the welfare of growing numbers of scab-infected sheep prompted vet Murray Corke to study the condition in a research project funded by animal health company Pfizer. In 1999, he sent a questionnaire to 5000 producer members of the National Sheep Association and received 1100 replies.
Responses indicated that scab was present in more than 10% of flocks questioned in west Scotland, south west Wales, Cornwall, Cumbria and Sussex. Although incidence was lower, there was also a clear band of infection across the midlands and into East Anglia.
"Assuming the survey is representative of the UK, sheep scab outbreaks are exceeding 5000 a year compared with 200 reported in 1992, the final year that sheep scab was notifiable."
Reasons for the increase are complex, says Dr Corke. "Dipping requirements relaxed and financial problems facing the industry meant corners were cut. Tight finances also mean sheep movements have increased, spreading scab, as producers try to make money."
Large flocks in hill areas were most at risk, with farms having fewer than 50 sheep at lower risk. "These may be regarded as hobby farms, but biosecurity measures, particularly fencing, are often good and commercial farms could learn lessons from these."
Producers believing they had scab used a number of treatments including organophosphorus or synthetic pyrethroid dips and injectables. "OP dipping was reported as the only treatment with 100% effectiveness, with about 90% of producers surveyed saying injectables were effective."
But Dr Corke believes the reason why injectables are reported as being less effective than dipping may lie in them being inappropriately or incorrectly used.
"Only one-third of suspected scab cases in the study were diagnosed by a vet. Scab can easily be confused with lice, so some cases reported as scab may have been lice. Injectables are not effective against chewing lice, so sheep continue to rub after treatment, which may be why producers thought injectables had failed to control scab."
His concern is backed up by Pfizer vet adviser, Steve Fay. "We always encourage producers to get a vet diagnosis as it can save money. Diagnosis costs only a few £s, whereas treating with an inappropriate product can cost much more."
As well as inappropriate use of injectables, there is also confusion over how the three injectable products licensed for scab control, doramectin, moxidectin and ivermectin, should be used. For scab prevention, one injection of one of the three products is required, whereas for control, one injection of doramectin or two injections of moxidectin or ivermectin are needed, says Mr Fay.
In his study, Mr Corke found that, on average, 1.6 injections of moxidectin and ivermectin were being given to control scab, indicating that some producers were only giving one injection. For doramectin, the average was 1.3 injections, suggesting that two were being given in some cases, when only one was required.
Underdosing can also be a problem, he adds. "Producers are good at assessing the weight of finished lambs, but not at estimating the weight of a breeding ewe."
Ideally, all ewes should be weighed to ensure accurate dosing, but weighing a sample and dosing to the highest weight is a practical alternative, he says.
Whatever treatment is used, sheep can become reinfected where biosecurity measures are inadequate, warns Dr Corke. "It can be hard to maintain good biosecurity in hill areas. Fencing around forests has not been maintained for many years, so there are feral sheep acting as a reservoir of infection."
Even within the same flock, problems can arise when sheep miss treatment, says Mr Fay. "Scab is the 100% disease. There is no point in only treating 99% of the flock, because untreated animals can reinfect the rest."
Introducing replacements to a flock also poses a risk and careful management is necessary, advises Dr Corke. "New sheep must be treated on entry and have no contact with the existing flock for at least 14 days. Different clothes and equipment should also be used for handling the two groups."
Scab remains devastating with severe welfare implications, says Dr Corke. "Eradication requires a collective approach because treatment on an individual farm basis will not control the problem." *